This invention concerns devices known as DBR's (distributed Bragg reflectors) and the techniques and equipment used to manufacture such devices. DBR's are optical fibers or other media that have been modified by modulating the longitudinal index of refraction of the fiber core, cladding or both to form a pattern. This pattern is generally known as a Bragg grating or image. A fiber equipped Bragg grating functions to modify the optical passband of the fiber (transmission characteristic) in such a way as to only transmit a narrow and controlled wavelength band.
Such DBR is typically a "lossless" device. That is, the sum of the optical transmission and reflection is unity for all wavelengths of interest. A telecommunication system equipped with DBR's can divide a single fiber into multiple channels (40, 80, or more) by dividing the full passband of the fiber into discrete channels which are assigned to specific wavelengths. An optical fiber laser having a DBR terminated optical cavity is described by G. A. Ball and W. W. Morey in "Continuously Tunable Single-Mode Erbium Fiber Laser", Optics Letters, 17 (1992) pp.420-422.
These DBR's are very useful in fiber-based telecommunications because of their high selectivity to wavelength (channel selection), their stability, and their packaging factor that allows them to directly couple to the telecommunications fiber and its pump amplifiers. Such a system which use DBR's for pump radiation reflection in a telecommunication application with optical pumping via doped fiber amplifiers is described in U.S. Pat. No. 5,218,655 (Mizrahi, et al). Remote sensing systems with DBR's use the passive and very sensitive nature of the DBR passband characteristics to sense various physical phenomenons (temperature, pressure, vibration, chemical content, etc.).
DBR devices are fabricated by exposure to "actinic" radiation in an appropriate spectral range (typically UV) much like photoresists and the like. This actinic exposure causes the index of refraction to permanently change in the exposed portion of the medium (i.e.,the core or cladding of the optical fiber. A period pattern can be formed on the surface of the fiber by superimposing two optical beams and forming an optical interference pattern. Two intersecting beams form a pattern whose grating period (distance between maxima) is given by =.lambda./((2*sin (.phi./2)) where .lambda. is the optical wavelength and .phi. is the angle between the beams. Sufficient exposure to UV radiation in such an interference pattern will produce a Bragg grating within a fiber and thereby, a DBR. A technique for fabricating such DBR's is described in U.S. Pat. No. 4,807,950 (Morey, et al).
DBR's fabricated with this technique are limited by several factors. First, the grating spacing is periodic. It is well known that quasiperiodic gratings (specifically "chirped" gratings) have preferred properties in certain applications. A chirped grating is a grating where the grating period changes (increases or decreases) monotonically down the fiber. Chirped gratings are useful in making broadband optical reflectors. Additionally, chirped gratings can be useful in removing undesirable wavelengths (and signals) as described in U.S. Pat. No. 5,625,472 (Mizrahi, et al).
U.S. Pat. No. 5,309,260 teaches that the normal exposure of DBR's made with the technique of U.S. Pat. No. 4,807,950 will suffer from exhibiting one or more subsidiary peaks or a regularly spaced series of peaks which may adversely affect the operation of telecommunication systems or remote sensing systems with active wavelength stabilization.
DBR fabrication techniques according to U.S. Pat. Nos. 4,807,950; 5,309,560; 5,388,173; 4,807,950; 5,625,472, and 5,694,248 all teach lateral imaging of the fiber. This lateral exposure has the effect of inducing a polarization birefringence in the fiber that is undesirable. Furthermore, these are inherently incompatible with specialty fibers such as tapered index fibers which may require further non-linearities in the grating period and index change profile as a function of the longitudinal position down the fiber. These DBR fabrication techniques are also highly labor intensive, subject to high levels of "trial and error", and therefore unsuited for high production environments. DBR's produced with these methods are, therefore, of low yield and must be subject to stringent quality control testing.
Known DBR fabrication techniques do not afford the DBR designer automatic and arbitrary control over the transmission and phase response characteristics of the imaging system for rapid production of custom DBR's.